"Devil comet" visible in night sky now won't swing by Earth again for decades
An unusual horned comet notable for a series of recent outbursts will be visible in the night sky for the remainder of March — and astronomers expect the so-called devil comet to make a rare appearance during the total solar eclipse on April 8.
Exactly why the dynamic comet takes on a shape that has drawn comparisons to the Millennium Falcon spacecraft from the "Star Wars" films when explosively active is still an enigma to scientists. But the celestial object only completes one orbit around the sun about every 71 years, similar to Halley's Comet, which make the odds of observing it for close study a once-in-a-lifetime opportunity.
Given that the comet won't pass by Earth again for decades, collective observations by astronomers could provide key insights into the true nature and behavior of Pons-Brooks.
Officially known as Comet 12P/Pons-Brooks, the celestial object will make its closest pass of the sun on April 21, coming within 74.4 million miles (119.7 million kilometers) of our star. The comet will then make its closest pass of Earth on June 2, but it will be 139.4 million miles (224.4 million kilometers) away from our planet and won't pose a risk.
For those in the Northern Hemisphere, the last 10 days of March will offer the best view, according to Dr. Paul Chodas, manager of the Center for Near-Earth Object Studies, and Davide Farnocchia, navigation engineer, at NASA's Jet Propulsion Laboratory in Pasadena, California.
"The comet will brighten a bit as it gets closer to the sun, and it should be visible to the naked eye low in the west about an hour after sunset," according to a joint email from Chodas and Farnocchia. "You should go to a location away from city lights and with an unobstructed view of the western horizon. It would be advisable to use a pair of binoculars, since the comet may be hard to locate without them."
After April 2, the comet is on track to move into the daytime sky and won't be visible to sky-gazers at night — but it will be visible when the moon's shadow temporarily blocks the sun's face from view on April 8.
"The comet would be located about 25 degrees away from the eclipsed sun," Chodas and Farnocchia said via email. "The comet should be fairly easy to find during the total solar eclipse, as well as a number of planets, but the main focus during those 4 minutes should be on the eclipse itself!"
After the comet's closest approach to the sun, known as perihelion, in late April the celestial body will shift to the southern night sky and only be visible to those in the Southern Hemisphere.
Two prolific discoverers, Jean-Louis Pons and William Robert Brooks, independently observed the devil comet for the first time in 1812. But the comet has likely made many trips around the sun over thousands of years, long before astronomers thought of comets as anything other than "something weird in the atmosphere," said Dr. Dave Schleicher, astronomer at Lowell Observatory in Arizona.
Astronomers estimate the massive comet to be between 6.2 to 12.4 miles (10 to 20 kilometers) in diameter, said astronomer Dr. Teddy Kareta, a postdoctoral associate at Lowell Observatory.
The rare visitor has a green appearance typical of most comets because they contain diatomic carbon molecules that absorb sunlight and emit a color that appears green from our perspective, Schleicher said.
A series of cosmic outbursts
Pons-Brooks recently captured the attention of astronomers after exhibiting intriguing behavior that caused the comet to have a horned appearance and soar through our solar system.
The comet has experienced a number of outbursts during the past eight months, causing it to eject gas and dust. While such releases are not uncommon in comets and a crescent or Pac-Man shape has been observed in other ones, it's difficult to tell what is normal for Pons-Brooks.
"I would say it's somewhat unusual in the number of outbursts it's been having," Schleicher said. "On the other hand, it's not like you have good records from the past to really let you know what is typical. And I suspect given the fairly large number of outbursts that have happened over the last eight months, that this is very clearly a usual occurrence for Pons-Brooks."
Comets are chunks of dust, rock and ice, essentially frozen remnants from the formation of the solar system. They also contain frozen elements such as carbon dioxide and carbon monoxide.
Comets heat up and brighten as they approach the sun, and some of the frozen gases stored in comets don't need to warm up much before they begin to turn into vapor, Schleicher said.
"We think the ultimate driver, of course, is heating from the sun," he said. "The comet is coming in; it's been sitting out in a deep freeze for years. The heat is going to be working its way from the surface down to wherever that carbon dioxide or carbon monoxide ice is located."
Astronomers suspect Pons-Brooks outbursts have occurred over the course of repeated events as heat vaporizes material inside the comet, which causes pressure to build up and break through the surface. While an explosion of gas wouldn't be visible in telescopes, the dust it kicks up would create the kind of events observed from Pons-Brooks, Schleicher said.
Scientists have traced the jets of material observed releasing from the comet during its outburst to two source regions on its surface. Astronomers are puzzled as to why "the whole surface isn't going off like mad," Schleicher said.
The observations imply that ice has crusted over the majority of the surface, or the ice has been vaporized away, leaving only dirt behind, but astronomers are "not quite sure which of those mechanisms runs the show," he said.
What we can learn from comets
An overlapping series of events likely has contributed to Pons-Brooks' distinctive look, but it could also be due to our perspective of the comet, Kareta said.
"These are three-dimensional objects," Kareta said. "When we take images of the night sky, we're taking them in a limited range of colors all flattened down in two dimensions. This will make things that might make perfect sense to you, if you're able to go up and walk around and see it in a couple of different perspectives, look much more complicated than they really are."
Astronomers are observing Pons-Brooks in the hopes of uncovering more details about its rotation rate, or the rate at which comets spin as they move through space. Pons-Brooks has a rotation period of 57 hours, which is longer than expected, and astronomers want to know if the jets of material releasing from the comet are speeding it up or slowing it down.
But Schleicher recommends keeping an eye out for the comet now rather than during the eclipse.
"In all my years, I've seen a lot of comets. I've only seen two total eclipses, and this will be No. 3. The first one I saw was back in 1991, from Baja. And that was just extraordinary. I remember realizing, no wonder this is considered the most magnificent sight in the heavens that any person on Earth can see. Get onto the path and see it in totality. You don't understand it until you've seen one."